My SIG 550 doesn't suffer from the "sticky mag" issue like my SG 540 does. My SIG also has a relatively soft Magazine Catch Spring that permits Trigger Finger depression sufficient to cleanly release a mag. Not so my FAMAE SG 540, which has a discernably stiffer Magazine Catch Spring that defies my weak and inadequate Trigger Finger. There is no way that I can hyper-extend the Mag Catch on the SG 540 with my Trigger Finger, and it must be hyper-extended to get a clean release with one press of the Mag Catch lever. I really think that the answer for the SG 540 lies in rounding out the sharp edges on the polymer mags as B-Wingpilot did to eliminate the need for "double clutching" (as Lone-Wolf so aptly dubbed it).
Speaking of Lone-Wolf, he turned me on to PR Precision who operate just down the road from me in Whitecourt, AB. Paul the proprietor designed and manufactured the SG540/542 Buttstock Adapter for the Zhukov Buttstock. I wanted to give one a try, so was very pleased to find that Paul had a few already machined and ready to ship for $150 plus GST and shipping ($178.50 all-in). I now have one enroute and will CeraKote it Armoury Black after it arrives. I haven't decided whether I want the Zhukov Buttstock on my SG 540 or my SAF .40 S&W. I reckon I will try it on both and keep it ono whichever I prefer. I may just go crazy and let one of my two OEM FAMAE Wire Folding Stocks go if I like the Zhukov form and function enough to keep it.
Thanks again to Lone-Wolf for the lead on PR Precision. If anyone else is looking to do the Zhukov folding stock on their SG 540/542, I recommend contacting PR Precision before too long. I'm not sure how many more adapters Paul has done up, nor do I know if he is planning to make more. So, if you want one now would be the time to act.
And thanks to Colin L for those most recent videos. I particularly enjoyed the Swiss training film!